A photographic process using silver halide has been widely used hitherto, because it is excellent in photographic characteristics such as sensitivity or control of gradation, etc., as compared with other photographic process such as electrophotography or diazophotography. In recent years, techniques capable of rapidly and easily obtaining image have been developed by modifying the image formation processing of light-sensitive materials (photographic materials) using silver halide by replacing the prior wet processing which uses a developing solution, etc., with a dry processing which uses heating, etc.
Heat-developable photographic materials are known in the field of this art. Heat-developable photographic materials and processes thereof have been described in, for example, Shashinkogaku no Kiso, published by Corona Co., pages 553-555 (1979), Eizojoho, page 40 (Apr. 1979), Nebletts Handbook of Photography and Reprography, 7th Ed. (Van Nostrand Reinhold Company), page 32, U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777 and Research Disclosure, pages 9-15, RD-17029(June 1978).
There are many known dry processes for obtaining color images. With respect to a process for forming color images by coupling an oxidation product of a developing agent with a coupler, p-phenylenediamine reducing agents and phenolic or active methylene couplers have been proposed in U.S. Pat. No. 3,531,286, p-aminophenol reducing agents have been proposed in U.S. Pat. No. 3,761,270, sulfonamidophenol reducing agents have been proposed in Belgian Pat. No. 802,519 and Research Disclosure, pages 31 and 32 (Sept. 1975), and combinations of sulfonamidophenol reducing agents and 4-equivalent couplers have been proposed in U.S. Pat. No. 4,021,240.
However, in these processes, the color images become impure, because reduced silver images and color images are formed at the same time on exposed parts after thermal development. Attempts at eliminating this problem include a process which comprises removing silver images by liquid treatment and a process which comprises transferring only dyes to another layer, for example, a sheet having a receiving layer. However, with these processes it is not easy to distinguish the dyes from unreacted substances and transfer only the dyes.
Further, a process which comprises forming a silver salt using a dye into which a nitrogen containing heterocyclic group is introduced and releasing the dye by thermal development has been described in Research Disclosure, pages 54-58, RD-16966 (May 1978). However, this process is not generally used because it is difficult to control the release of dyes in areas which were not exposed to light and, consequently, clear-cut images cannot be obtained.
With respect to a process for forming positive color images by a heat-sensitive silver-dye bleaching process, useful dyes and a method of bleaching have been described in, for example, Research Disclosure, pages 30 to 32, RD-14433 (April 1976), and pages 14-15, RD-15227 (December 1976), and U.S. Pat. No. 4,235,957, etc.
In this process, however, it is necessary to use extra steps and materials, for example, heating is carried out by superposing a sheet containing an activating agent in order to promote bleaching of the dyes. Further, the resulting color images are gradually reduced and bleached by coexistent free silver during preservation for a long period of time.
Further, a process for forming color images using leuco dyes has been described in, for example, U.S. Pat. Nos. 3,985,565 and 4,022,617. However, with this process the photographic materials gradually color during preservation, because the leuco dyes are difficult to contain in the photographic materials.